Can magnesium sulfate be administered intramuscularly (IM)?

Intramuscular Administration of Magnesium Sulfate

Yes, magnesium sulfate can be administered intramuscularly (IM), particularly in specific clinical scenarios such as eclampsia treatment where the FDA-approved drug label specifically recommends this route of administration.

Approved Indications for IM Magnesium Sulfate

Pre-eclampsia and Eclampsia

• In severe pre-eclampsia or eclampsia, IM administration is a well-established route 1
• The Pritchard regimen used in the MAGPIE trial involves:
  • Loading dose: 4g IV followed by 5g in each buttock (total 14g)
  • Maintenance: 5g IM every 4 hours in alternate buttocks for 24 hours 2
• The undiluted 50% solution is appropriate for IM administration in adults 1

Magnesium Deficiency

• For mild magnesium deficiency: 1g (2mL of 50% solution) IM every six hours for four doses 1
• For severe hypomagnesemia: up to 250mg per kg of body weight (0.5mL of 50% solution) IM within a four-hour period 1

Administration Considerations

Concentration and Dilution

• For adults: Deep IM injection of the undiluted (50%) solution is appropriate 1
• For children: The solution should be diluted to 20% or less concentration prior to IM injection 1

Injection Technique

• IM injections should be administered deep into the gluteal muscle (buttocks) 2, 1
• For eclampsia treatment, alternate buttocks for repeated injections 2, 1

Monitoring

• Clinical monitoring is essential during magnesium sulfate administration:
  • Respiratory rate (should be ≥12/min)
  • Deep tendon reflexes (should be present)
  • Urine output (should be ≥30 mL/hour) 3
• Serum monitoring is not necessary if clinical monitoring is adequate 3

Pharmacokinetics of IM Administration

• After IM administration, magnesium is absorbed into the bloodstream with approximately 40% binding to plasma proteins 4
• The unbound magnesium diffuses into extravascular-extracellular space, bone, and across the placenta 4
• Magnesium is almost exclusively excreted in the urine, with 90% eliminated within 24 hours 4

Cautions and Contraindications

• Maximum dosage in severe renal insufficiency: 20g/48 hours with frequent serum magnesium monitoring 1
• Continuous use in pregnancy beyond 5-7 days can cause fetal abnormalities 1
• Use with caution in patients receiving digitalis or neuromuscular blocking agents 5
• Signs of magnesium toxicity include:
  • Loss of patellar reflex (at 3.5-5 mmol/L)
  • Respiratory depression (at 5-6.5 mmol/L)
  • Cardiac conduction abnormalities (>7.5 mmol/L)
  • Cardiac arrest (>12.5 mmol/L) 4

Alternative Routes of Administration

• IV administration is preferred when resources permit, as it has fewer injection site problems 3
• Subcutaneous administration has been reported as effective in case reports for chronic hypomagnesemia management, though this is an off-label route 6
• For acute severe asthma exacerbations, IV administration (2g over 20 minutes) is the recommended route rather than IM 5

Clinical Pearls

• IM administration may be particularly valuable in resource-limited settings where IV administration capabilities are limited 2, 3
• Task-shifting policies in some regions allow lower-level providers to administer IM magnesium sulfate (5mg in each buttock) and refer patients 2
• Visual disturbances including blurred vision and diplopia are common side effects during magnesium sulfate administration 7